Mechanical and chemical-mechanical planarizing processes (collectively "CMP") are used in the manufacturing of microelectronic devices for forming a flat surface on semiconductor wafers, field emission displays, and many other microelectronic substrates. FIG. 1 schematically illustrates a planarizing machine 10 with a platen or table 20, a carrier assembly 30, a polishing pad 21 positioned on the table 20, and a planarizing fluid 23 on the polishing pad 21. The planarizing machine 10 may also have an under-pad 25 attached to an upper surface 22 of the platen 20 for supporting the polishing pad 21. In many planarizing machines, a platen drive assembly 26 rotates (arrow A) and/or reciprocates (arrow B) the platen 20 to move the polishing pad 21 during planarization.
The carrier assembly 30 controls and protects a substrate 80 during planarization. The carrier assembly 30 typically has a substrate holder 32 with a pad 34 that holds the substrate 80 via suction. A carrier drive assembly 36 typically rotates (arrow C) and/or translates (arrow D) the substrate holder 32. The substrate holder 32, however, may be a weighted, free-floating disk (not shown) that slides over the polishing pad 21.
The combination of the polishing pad 21 and the planarizing fluid 23 generally defines a planarizing medium 28 that mechanically and/or chemically-mechanically removes material from the surface of the substrate 80. The polishing pad 21 may be a conventional polishing pad composed of a polymeric material (e.g., polyurethane) without abrasive particles, or it may be an abrasive polishing pad with abrasive particles fixedly bonded to a suspension material. In a typical application, the planarizing fluid 23 may be a CMP slurry with abrasive particles and chemicals for use with a conventional non-abrasive polishing pad. In other applications, the planarizing fluid 23 may be a chemical solution without abrasive particles for use with an abrasive polishing pad.
To planarize the substrate 80 with the planarizing machine 10, the carrier assembly 30 presses the substrate 80 against a planarizing surface 24 of the polishing pad 21 in the presence of the planarizing fluid 23. The platen 20 and/or the substrate holder 32 then move relative to one another to translate the substrate 80 across the planarizing surface 24. As a result, the abrasive particles and/or the chemicals in the planarizing medium 28 remove material from the surface of the substrate 80.
CMP processes must consistently and accurately produce a uniformly planar surface on the substrate to enable precise fabrication of circuits and photo-patterns. Prior to being planarized, many substrates have large "step heights" that create a highly topographic surface across the substrate. Yet, as the density of integrated circuits increases, it is necessary to have a planar substrate surface at several stages of substrate processing because non-uniform substrate surfaces significantly increase the difficulty of forming sub-micron features or photo-patterns to within a tolerance of approximately 0.1 .mu.m. Thus, CMP processes must typically transform a highly topographical substrate surface into a highly uniform, planar substrate surface (e.g., a "blanket surface").
In the competitive semiconductor industry, it is desirable to maximize the throughput of CMP processing by producing a blanket surface on a substrate as quickly as possible. The throughput of CMP processing is a function of several factors, one of which is the ability to accurately stop CMP processing at a desired endpoint. In a typical CMP process, the desired endpoint is reached when the surface of the substrate is a blanket surface and/or when enough material has been removed from the substrate to form discrete components on the substrate (e.g., shallow trench isolation areas, contacts, damascene lines, etc.). Accurately stopping CMP processing at a desired endpoint is important for maintaining a high throughput because the substrate may need to be re-polished if the substrate is "under-planarized." Accurately stopping CMP processing at the desired endpoint is also important because too much material can be removed from the substrate, and thus the substrate may be "over-polished." For example, over-polishing can cause "dishing" in shallow-trench isolation structures, or over-polishing can completely destroy a section of the substrate. Thus, it is highly desirable to stop CMP processing at the desired endpoint.
In one conventional method for determining the endpoint of CMP processing, the planarizing period of one substrate in the run is estimated using the polishing rate of previous substrates in the run. The estimated planarizing period for a particular substrate, however, may not be accurate because the polishing rate may change from one substrate to another. Thus, this method may not accurately planarize all of the substrates in a run to the desired endpoint.
In another method for determining the endpoint of CMP processing, the substrate is removed from the pad and the substrate carrier, and then a measuring device measures a change in thickness of the substrate. Removing the substrate from the pad and substrate carrier, however, is time consuming and may delay further processing until the measurement is completed. Thus, this method generally reduces the throughput of CMP processing.
In still another method for determining the endpoint of CMP processing, a portion of the substrate is moved beyond the edge of the pad, and an interferometer directs a beam of light directly onto the exposed portion of the substrate and through a layer of the substrate to detect a thickness of the layer. The substrate, however, may not be in the same reference position each time it overhangs the pad. For example, because the edge of the pad is compressible, the substrate may not be at the same elevation for each measurement. Thus, this method may inaccurately measure the change in thickness of the layer of the wafer.
In yet another method for determining the endpoint of CMP processing, U.S. Pat. No. 5,036,015 discloses detecting the planar endpoint by sensing a change in friction between a wafer and the polishing medium. Such a change in friction may be produced by a different coefficient of friction at the wafer surface as one material (e.g., an oxide) is removed from the wafer to expose another material (e.g., a nitride). In addition to the different coefficients of friction caused by a change of material at the substrate surface, the friction between the wafer and the planarizing medium generally increases during CMP processing because more surface area of the substrate contacts the polishing pad as the substrate becomes more planar. U.S. Pat. No. 5,036,015 discloses detecting the change in friction by measuring the change in current through the platen drive motor and/or the drive motor for the substrate holder.
In still a further method for determining the endpoint of CMP processing, such as is disclosed in U.S. Pat. No. 5,559,428, the chemical composition of the CMP slurry is analyzed to determine when a layer of a first material has been removed to expose a layer of a second, different material. For example, planarization may continue through the first material until the second material is exposed, at which point some of the second material is removed and enters the slurry. The second material and the slurry is identified using instrumentation such as inductively coupled plasma for atomic emission spectroscopy, and the planarization process is halted.
In yet another method for determining the endpoint of CMP processing, such as is disclosed in U.S. Pat. No. 5,321,304, the substrate includes parasitic conductive contacts or terminals buried beneath an insulative layer. When the insulative layer over the contacts has been removed by planarization, the terminals come into electrical contact with probes in the polishing pad or the planarizing liquid, completing an electrical circuit and indicating the endpoint. A drawback with this method is that it is not suitable for determining the endpoint of a conductive layer.
In still a further method for determining the endpoint of CMP processing, such as is disclosed in U.S. Pat. No. 4,793,895, the substrate includes contacts or terminals positioned beneath an insulative layer and coupled to each other through circuitry internal to the substrate. The insulative layer is placed face down against a polishing pad that has two corresponding terminals coupled to a power source. When the insulative layer has been removed by planarization, the terminals in the polishing pad make electrical contact with the terminals of the substrate, completing an electrical circuit and indicating that the endpoint has been reached. The method can also be used to monitor the resistivity of a conductive layer. One drawback with this method is that at least one terminal is positioned in the polishing pad where it can become contaminated with slurry and other particulates. Another drawback with this method is that a conductive path must be maintained between the substrate and the polishing pad, which may be difficult during planarization because the substrate and the polishing pad move relative to one another.